Summary

Instead of:

if c in "abcdefghijklmnopqrstuvwxyz":
    print(c)

Use:

from string import ascii_lowercase
if c in ascii_lowercase:
    print(c)

And more generally:

• abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ -> string.ascii_letters
• abcdefghijklmnopqrstuvwxyz -> string.ascii_lowercase
• ABCDEFGHIJKLMNOPQRSTUVWXYZ -> string.ascii_uppercase
• 0123456789 -> string.digits
• 0123456789abcdefABCDEF -> string.hexdigits
• 01234567 -> string.octdigits

The main reason is, when using a string literal like abcdefghijklmopqrstuvwxyz it's not abvious when a letter is missing (oops, just did it, did you see it?), so it's a potential source of errors.